Prior treatment (priming) with a sub-immunogenic dose of Type III pneumococcal polysaccharide (SSS-III) results in the development of an antigen-specific state of unresponsiveness termed low-dose paralysis. Such unresponsiveness can be transferred by spleen cells obtained from mice within 5-24 hrs after priming; the suppressive activity of primed cells is abolished by treatment with monoclonal antibody specific for the Thy 1.2 alloantigen in the presence of complement. These findings clearly show that low-dose paralysis is mediated by thymus-derived (T) suppressor cells.